The invention relates to a method for calculating and producing a computer-generated hologram and also a storage medium with a computer-generated hologram and a reading device.
Computer-generated holograms are two-dimensional holograms which comprise individual pixels with different optical properties and from which images and/or data are reproduced by diffraction when they are illuminated with a coherent electromagnetic wave, in particular a light wave, in transmission or reflection. The different optical properties of the individual pixels may be reflection properties, for example as a result of surface topography, varying optical path lengths in the material of the storage medium (refractive index) or color values of the material.
The optical properties of the individual pixels are calculated by a computer, so they are so-called computer-generated holograms (CGH). With the aid of a focused writing beam, the individual pixels of the hologram are written into the material of the storage medium during the writing of the hologram, the focus lying in the region of the surface or in the material of the storage medium. Focusing brings about a small area of action on the material of the storage medium in the region of the focus, so that a large number of pixels of the hologram can be written in a small region. The optical property of the pixel respectively written in this case depends on the intensity of the writing beam. For this purpose, the writing beam is scanned in two dimensions with varying intensity over the surface of the storage medium or within the volume, for example in a layer provided with a transparent protection. The modulation of the intensity of the writing beam takes place in this case either by means of an internal modulation of the light source, for example a laser diode, or by means of an external modulation of a writing beam outside the light source, for example with the aid of optoelectronic elements. In addition, the light source may be formed as a pulsed laser, the pulse lengths of which can be controlled, so that control of the pulse energy of the writing beam can take place on the basis of the pulse lengths.
Consequently, the scanning of the intensity-modulated writing beam produces one or more areas with an irregular pixel distribution, the computer-generated hologram or holograms. This can be used for identifying and individualizing any objects desired.
Scanning lithographic systems for producing computer-generated holograms are known per se. Both laser-lithographic and electron-beam lithographic systems are meant here, or other lithographic systems. Laser-lithographic systems are capable of achieving a resolution of approximately 25,000 dpi on an area of 1 to 25 mm2. The writing speed is approximately 1 Mpixel/s, so that in each case a hologram can be written in a time of about 1 s.
Each computer-generated hologram can be read out only with the aid of a reading beam; the surface of the hologram itself has no directly visible and perceptible information. For this reason, it has so far been difficult to introduce and popularize computer-generated holograms in application areas in which information which can be read out directly, for example 1D or 2D barcodes, has been used. This is because a system change from one reading technique to another new reading technique always requires a transitional period in which both applications can be used simultaneously. The invention is therefore based on the technical problem of further improving the calculation and production of a computer-generated hologram, so that a combination with directly perceptible images in the structure of the pixel distribution is made possible. Similarly, the technical problem is that of providing a storage medium which can be provided with a combined pixel distribution, and also of providing a corresponding reading device.